This invention concerns a method for processing workpieces by ultrasonic energy and, more specifically, refers to a method for processing thermoplastic workpieces using vibratory energy in the ultrasonic frequency range for bonding, sealing, or welding thermoplastic film and fabric materials as well as substantially rigid workpieces. Quite specifically, this invention concerns a method wherein the motional amplitudes and engaging forces of the ultrasonic transducer horn in contact with the workpiece are varied over particular profiles during the weld cycle, thereby varying the power transmitted from the horn to the workpiece and the engaging force during such cycle.
Ultrasonic welding is one of the most common techniques for joining thermoplastic sub-assemblies. Its primary advantages are its short cycle times and moderate capital costs. Typical manual cycle production times are less than three to five seconds, resulting in production rates above 500 units per hour. The traditional techniques of welding thermoplastic workpieces and plunge sealing film and fabric materials by ultrasonic energy are well known. The techniques work by applying relatively high stresses to the parts being joined to induce hysterisis heating at the bond line. During a weld cycle, the workpieces are supported on an anvil. A horn, dimensioned to be resonant, preferably as a one-half wavelength resonator or multiples thereof, for high frequency vibrations of predetermined frequency traveling longitudinally therethrough, is brought into forced engagement with the workpiece for the duration of the weld cycle, and responsive to the horn being rendered resonant, ultrasonic energy is transmitted to the workpieces, for causing a softening and flowing of the thermoplastic material.
Generally it is recognized that the ultrasonic energy or power transmitted to the workpiece is dependent on three factors, namely, the frequency of the electroacoustic transducer, the engaging force or clamping pressures applied to the workpiece by the horn, and the motional amplitude of the horn as it transmits the energy to the workpieces. It will be appreciated that, in general, in an ultrasonic welding machine, the frequency of the electroacoustic transducer is relatively constant, preferably within the range of 20-40 KHz. Similarly, in the past, it has been the common practice to retain the motional amplitude of the horn, i.e., the peak-to-peak mechanical excursion of the frontal horn surface in contact with the workpieces (measured in microns, and herein designated as ".mu.m.sub.pp "), constant during the entire weld cycle.
In the welding of rigid thermoplastic components, the ultrasonic energy transmitted to the weld surfaces from the horn propagates through the upper workpiece, and is concentrated at the weld surfaces by means of an "energy director," as described in the coassigned U.S. Pat. No. 4,618,516. The energy director is a molded-in stress concentrator which locally deforms under the motional force and stress induced by the ultrasonic energy. The local deformation of the energy director initiates heating and melting from the hysterisis losses of the thermoplastic. The average heating rate (Q.sub.avg) of the energy director is governed by the general equation: ##EQU1## where .epsilon..sub.O is the strain, which is proportional to amplitude; .omega. is the frequency; and E" is the complex loss modulus.
Once melting occurs, the molten energy director flows across the surface to be joined, forming a weld bead. The rate of flow is determined by a number of variables, but is primarily effected by the temperature of the melt and the engaging force applied to the parts. After the application of ultrasonic energy is discontinued, the melt solidifies under a continued engaging force to form a fused joint, thereby establishing a bond or weld between the workpieces. It can be seen in the average heating rate equation set forth above that the heating is proportional to the square of the applied strain, which is in turn proportional to the vibrational amplitude of the horn face. Thus, the bond line heating can be controlled by varying the motional amplitude. At higher amplitudes, the average bond line heating rate is higher, which in turn causes the temperature to rise to higher levels, resulting in the melt having a higher flow rate. High flows rates lead to a high degree of molecular alignment, but this alignment is orthogonal to the desired loading stress on the workpieces, and can result in fractures initiated at any discontinuities in the weld. An additional effect of high flow rates is significant flash, which is undesirable as a cosmetic consideration. On the other extreme, insufficiently high motional amplitude can result in a non-uniform melt initiation and/or premature solidification of the melt due to inadequate heating.
Similarly, it is seen that the strength of the resulting weld is generally inversely proportional to the engaging force. A higher engaging force promotes a high degree of molecular alignment within in the melt by forcing the molten material to flow and solidify under the higher pressures. This results in a weaker weld upon solidification, subject to fractures as described above. The level of the engaging force additionally is seen to effect the time required to complete the weld cycle. At the higher engaging forces, the melted material flows more rapidly, and is reduced to a thinner layer, allowing for a more rapid solidification. Lower engaging forces permit the melt to form a thicker layer, and decrease the flow rate. Accordingly, the weld cycle time is proportional to the engaging force.
Conventional methods of ultrasonic welding do not permit the simultaneous control and variation of the motional amplitude and engaging force during the weld cycle. Rather, it has been common practice to maintain the motional amplitude of the horn, i.e. the peak-to-peak mechanical excursion of the frontal horn surface in contact with the workpiece, constant at a rate sufficient to produce the desired flow rate in the molten material during the entire time interval of ultrasonic power transfer to the workpiece. Similarly, most welding systems are pneumatically driven by standard pressure regulators which hold the engaging force between the frontal horn surface and the workpiece relatively constant during the weld cycle. However, during an ultrasonic thermoplastic welding cycle, the workpieces transition through several different phases, each of which may benefit from different motional amplitudes and engaging forces to decrease weld cycle time and increase the quality of the weld in terms of strength, consistency, and cosmetics. Accordingly, the present invention discloses a method in which both the motional amplitude of the horn and the engaging force are varied during the weld cycle in response to a control signal which may be responsive, for instance, to a change in the power transmitted from the horn to the workpiece, a process related change of the workpiece dimensions, a process related timing signal, or some other process related parameter.
The invention, which will be described in detail hereinafter, has been made possible by the development of a method of varying motional amplitude during ultrasonic welding disclosed in U.S. Pat. No. 5,434,863 issued to J. L. Frantz, dated Jul. 25, 1995, entitled "Method for Processing Workpieces by Ultrasonic Energy", which patent is specifically incorporated herein for reference. This method describes the process of reducing motional amplitude of the horn during a weld cycle, thereby varying the power delivered to the workpieces and producing a stronger weld.